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Employee Share-Based Incentive Compensation Plans (Performance Shares Granted Under the WTM Incentive Plan) (Details) - White Mountains - Performance Share - W T M Incentive Plan - USD ($) $ in Millions | Sep. 30, 2018 | Jun. 30, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Dec. 31, 2016
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 41,235 |  |  |  |  |
Accrued Expense | $ 35.4 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (619) |  |  |  |  |
Accrued Expense, Assumed forfeitures | (0.5) |  |  |  |  |
Total Target Performance Shares Outstanding | 40,616 | 40,616 | 50,515 | 50,319 | 50,575 | 80,353
Total Accrued Expense | $ 34.9 | $ 27.2 | $ 45.8 | $ 37.8 | $ 31.2 | $ 42.4
Performance Cycle 2016 to 2018 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 13,715 |  |  |  |  |
Accrued Expense | $ 18.9 |  |  |  |  |
Performance Cycle 2017 to 2019 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 14,070 |  |  |  |  |
Accrued Expense | $ 13.3 |  |  |  |  |
Performance Cycle 2018 to 2020 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 13,450 |  |  |  |  |
Accrued Expense | $ 3.2 |  |  |  |  |

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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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